This invention relates to self-clocking glyph codes for recording digital information on graphic recording media, such as plain paper. Even more specifically, this invention pertains to techniques for increasing the security of the digital information while preserving the homogenous visible appearance of the resultant product.
Distinguishing copies or counterfeits of original authentic documents is a common important objective. Printing documents with features that are difficult to copy or counterfeit is one general approach to counterfeit suppression. Excellent advanced image copying technology, however, makes it more difficult to print image characteristics that are adequately copy-proof. Special physical printing, such as holograms, can be employed. However, this approach requires special printing equipment and may be counterfeited if that equipment is accessible to the counterfeiter.
Covert chemical taggants are known and have been incorporated into print material such as paper and/or toner. However, counterfeits could be made if the materials making up the chemical taggants could be obtained and applied in the required pattern. Such copying is facilitated if the taggants occur in a distinguishable manner, that is, if the taggants are visually detectable or capable of being detected by optical machine, such as is the case when using tagged yellow toner as a taggant.
Robust self-clocking glyph codes are known. Such codes can generally encode arbitrary digital message codes up to a certain capacity, typically several hundred bytes per square inch. Glyph codes typically comprise distinguishable rotated marks; for example, 3,600 such marks per square inch may form a rectangular lattice. These glyph codes can include encrypted messages, including digital signatures such as those used in secure electronic communications. However, conventionally printed glyph codes are readily copyable, thereby reproducing their information content. Simply printing glyph codes with a taggant material can be counterfeited if the taggant material can be obtained.
Accordingly, an object of the present invention is to provide a record and related method for encoding information which cannot easily be copied or otherwise counterfeited.